1. Field of the Invention
The present invention relates to a method and a device for focus adjustment of an optical writing unit which performs electrophotographic image forming operation, in which an electrostatic latent image is formed on a surface of an image carrying member by projecting light modulated by image data onto the image carrying member. The invention also relates to an image forming apparatus incorporating such a focus adjustment device.
2. Description of the Related Art
An image forming apparatus, such as a copying machine or a laser printer, for performing electrophotographic image forming operation forms an electrostatic latent image on a surface of an image-carrying member (photosensitive drum) by projecting light modulated by digitized image data from light-emitting elements of an optical writing unit, develops a visible image from the latent image by means of toner particles, and transfers the visible image onto a printing medium such as a sheet of blank paper. Optical writing units used in such image forming apparatuses are classified into two types: a laser scanning type and a solid-state light source scanning type.
An optical system of the laser scanning type optical writing unit needs to have a long light path since it deflects a light beam emitted from a single laser light-emitting device over a wide scanning angle by means of a spinning polygon mirror, for example. This structure makes it difficult to reduce the size and cost of the image forming apparatus employing the laser scanning type optical writing unit.
On the other hand, the solid-state light source scanning type optical writing unit employs an array of light-emitting elements, such as light-emitting diodes (LEDs) or electroluminescent (EL) segments, and an array of lenses, such as selfoc (self-focusing) lenses, for converging light emitted from the light-emitting elements and projecting the converged light onto a surface of an image-carrying member. To form an A3 size image at a resolution of 600 dots per inch (dpi), for example, the number of the light-emitting elements to be arranged in a line is approximately 7000. In the solid-state light source scanning type optical writing unit, each of the light-emitting elements is used to write one pixel on the image-carrying member, so that the light path length of its optical system can be shortened, making it possible to reduce the size and cost of the image forming apparatus. Accordingly, the solid-state light source scanning type is an industrial mainstream of the optical writing units in recent years.
In the solid-state light source scanning type optical writing unit, too short a light path length is likely to decrease the depth of focus, resulting in loss of focus. This defocusing problem can be solved by precisely adjusting the distance between the optical writing unit and the image-carrying member. Therefore, a worker in an image forming apparatus assembly line visually examines printed images and manually adjusts the distance between the optical writing unit and the image-carrying member repeatedly on a trial-and-error basis.
Adjustment of the distance between the optical writing unit and the image-carrying member performed by manual operation in this fashion is fairly complicated and difficult, requiring skilled workers and long work time.
A conventional technique related to this kind of focus adjustment procedure is disclosed in Japanese Laid-open Patent Publication No. S62-166372. According to the disclosure, an image is first formed with an optical writing unit held at a specific slant angle with respect to an image-carrying member so that the focal length varies along an array of light-emitting elements and, after displacing the optical writing unit parallel to its original position without changing its slant angle, an image is formed again. Then, the slant angle of the optical writing unit with respect to the image-carrying member and the amounts of offset of the focal length are calculated from information on the positions of two pixels best focused in the two successive image forming processes.
Japanese Laid-open Patent Publication No. H7-270673 discloses another conventional technique, in which image patterns are formed while varying the focal length and repeatedly turning on and off an optical writing unit whereby the optical writing unit is set to a position where an image of the lowest density is obtained.
However, the technique disclosed in Japanese Laid-open Patent Publication No. S62-166372 involves the need to perform a complex mathematical operation for focus adjustment. Also, Japanese Laid-open Patent Publication No. H7-270673 is intended for use in an apparatus employing an image-carrying member for producing a binary (black and white) image and there is no mention of a focus adjustment technique for an image-carrying member for multi-valued image forming applications.